Fugro-LCT Gravity and Magnetics

There is an increasing number of marine 3D seismic surveys being acquired with concurrently recorded high resolution gravity and magnetics data. Instrumentation, navigation, and processing advances have led to significantly increased gravity and magnetics data resolution. State of the art workstation software tools provide a means for the integration of seismic, gravity, and magnetic data. Interpretations have aided in the delineation of complex imaging problems including the verification and enhancement of seismic velocity models. A review is given of technical developments, economic considerations, and case studies in support of integrated 3D marine surveys utilizing gravity, magnetics, and seismic data.

Using new interpretation case studies from the Gulf of Mexico and offshore Europe, it is possible to gain insights into enhancing and constraining the interpretation of 3D seismic data using gravity and magnetics methods. Using digital horizons interpreted from a 3D seismic volume and high resolution gravity and magnetics data, an integrated and constrained 3D geologic model can be quickly built and tested.

The acquisition of high resolution gravity and magnetic data in conjunction with 3D seismic surveys is now an accepted norm in Europe, and is quickly gaining acceptance in the Gulf of Mexico. Over 300 OCS blocks of 3D-acquired gravity and major areas of high resolution aeromagnetic data have been and are now being recorded in the Gulf of Mexico. The use of high resolution gravity in seismic velocity analysis and the use of velocity grids for localized, focused density input to gravity models is now possible. A detailed example of a localized conversion of a velocity cube to a density volume is provided from the Southern Additions, offshore Louisiana.

A brief review of instrumentation, processing techniques, costs, and integrated software applications is provided to set the framework for the interpretation case studies. Gravity and magnetic instrumentation has decreased in size and increased in sampling and resolution power. Processing of the data using high quality DGPS positioning data has resulted in a dramatic increase in the resolution of shipborne gravity. Workstation applications are now in use which facilitate the direct transfer of data and models between seismic and gravity/magnetic modeling software systems.

With the development of digitally controlled marine gravity systems as described by LaFehr et al1,2 the restrictions of hardware-defined filtering have been removed. This is a major factor in recovering maximum signal in final processed results, by allowing the data processor to quantitatively determine optimum filter parameters for specific sea conditions and induced noise levels. Marine surveys are now routinely producing results of 0.1-0.5 mgal resolution over 500 to 1000 meter minimum wavelengths. This change reflects a) the new digital gravity meter technologies, b) benefits of DGPS positioning for the removal of vehicular motion effects on the gravity meter, and c) the benefit of larger, more stable multi-streamer seismic vessels. In addition, the better spatial sampling of the data due to closely spaced ship tracks on a 3D seismic survey greatly increase the data resolution over 2D methods (Fig. 1).

Likewise in magnetics, increases in resolution have also occurred, and are due to a combination of better equipment, more frequent data sampling, and enhanced data processing techniques.

Closely-sampled 3D-acquired potential fields data has presented new challenges in data processing. High resolution results Fig.2 and (Fig. 3) have required the development of new line leveling algorithms and filtering techniques to address low amplitude random noise in the data when profiles are combined to produce grid results. Predictive gridding and narrow band Weiner strike filtering are some of the techniques now employed.

With the typical 3D survey vessel pre-equipped with DGPS navigation, power, and space, little additional cost is incurred in the addition of high-resolution gravity and magnetics to the 3D survey effort. Worldwide, the cost of acquiring and processing this data is on the order of one percent to three percent of the seismic data acquisition (before seismic processing) costs (Fig.4).

As explorationists, we are all familiar with the important industry trend of integration. This term has been used to describe many things. It comes from the same root as the word integritymeaning the state of being truthful or whole. In exploration it is used to describe the incorporation of well data, geology, seismic, gravity, magnetics, cultural, and other data to form a whole or integrated model of the subsurface. 3D-acquired gravity and magnetic data is playing a larger role than ever in finding oil in the Gulf of Mexico and elsewhere in the world, through the use of truly integrated subsurface models and interpretations.

As described by Saad (3) and Pawlowski (4), a breakthrough in effective integration has been the emergence of workstation applications for simultaneous modeling of seismic, gravity, and magnetic data.(Fig.5)

Team-oriented Exploration Tools. With the trend towards highly focused exploration teams, the smooth interaction and coupling of multiple geophysical disciplines is essential. Explorationists are expected to employ and be familiar with more disciplines on a continuing basis. The development of workstation applications which enable the interpreter to simultaneously refine the subsurface model using seismic, gravity, and magnetic data has been a giant step forward.

Data Applications. The benefits of acquiring and incorporating 3D-acquired gravity and magnetic data into subsurface models is multifold:

Even with good quality 3D seismic data, interpreters can have problems in defining the salt/sediment boundary at the flanks of a salt dome, salt sheet, or other complex structure. For decades, gravity has been used to address this problem. The most recent changes are: a) better acquisition technology and processed data, and b) truly integrated workstation software tools. By incorporating a co-recorded data set, independently measuring a related property of the subsurface (density from gravity and velocity from seismic), the interpreter can place a much higher degree of confidence in the final geologic interpretation. To quantify this observation, the following case studies show that incorporating 3D seismic with high resolution gravity and magnetics can alter the base of salt interpretation by several thousand feet from the 3D seismic interpretation alone. In some cases, results from gravity modeling have provided excellent insights into the geology below a salt body, enabling the seismologists to refine their migration velocity model for the structure, and as a result, refine the seismic image through reprocessing the data using the new velocity model.

Velocity Modeling. At present gravity is commonly incorporated into the subsurface model after the seismic data has been: 1) fully processed, 2) specialty processed, 3) migrated, and 4) interpreted. Present work is underway to incorporate the gravity and magnetic data into the seismic data at a much earlier stage, ideally during the velocity analysis process. The end result will be a velocity model which respects the constraints of the gravity and magnetic data, and a much more refined density model (from seismic velocities) for use in the interpretation of the gravity and magnetic data.

Case Study 1: Gulf of Mexico Velocity-Density Volume and Deep Low Density Zone Mapping

As described by Bain et al(5), the primary determining factor in gravity interpretation validity is the amount and accuracy of density data. In the same way, magnetic interpretation is limited by magnetic susceptibility control. In this case study, data from 54 check shot velocity surveys with co-located gamma-gamma density logs are analyzed to determine a localized empirical relationship between near-surface seismic velocity and density for the Southern Additions, offshore Louisiana. When these data are plotted (Fig. 6) versus the more commonly used Gardner's Equation for density/velocity conversion, it is apparent that the empirically-derived LASA equation provides a more suitable velocity to density conversion for their data in this area. With the aid of this relationship, and density logs from over 1,500 wells, a 32-layer (stacked grids) density model is constructed for the Southern Additions, for use in the regional and prospect level mapping of inter-salt sediment thicknesses, and relief of the deep low density zone (Fig. 7). The LASA velocity density relationship is most useful in the upper 5,000 feet of the subsurface model. In this area, the impact of incorrect densities on the modeling results is greatest. It is also the area where density logs are most lacking.

Effective use of velocity data enhances density control for gravity modeling. Classical gravity and magnetics modeling has often been performed using a single density value for each geologic unit or "layer" in the model. We compare a map view of an interval seismic velocity grid (Fig.8) for a 2,000 foot layer of an area offshore Louisiana, with the corresponding density grid as computed form the velocities using Gardner's Equation (Fig.9). The significant lateral variations indicate that for the most accurate modeling, laterally varying interval densities should be used as a better approximation of geologic truth than a fixed value per layer.

The tabular salt body shown in Fig. 10 is interpreted from a 3D seismic survey. As with many salt features the top of the body is easily interpreted (except where steeply dipping) but the base of salt and the "Gumbo Zone" below the salt are difficult, if not impossible, to interpret from the seismic data alone. Furthermore, the time based seismic interpretation does not provide depth information important to the development of a successful interpretation. A real-time integrated modeling technique using gravity, well log and seismic data is conducted in order to:

An initial 2D depth model is generated using the seismic derived salt geometry and sediment velocities. Laterally varying density data obtained from the 3D density volume discussed in Example 1, and a salt density of 2.08 g/cm3 are used to constrain the model (Fig. 11). The calculated gravity of the initial model has a similar shape to the observed gravity data. However, the significant difference in the magnitudes of the calculated and observed gravity fields indicates that the initial interpretation is not entirely correct. The fit of the calculated field more closely matches that of the observed field after:

Conversion of the salt body from 2D to 2.5D. Seismic derived top salt maps were used to determine approximate half-widths for the salt body to a depth of 8,000 feet. The salt body has significant strike length below 8,000 feet to be treated as a 2D body.

Integration of density data derived from well logs with sediment warping derived from the seismic image. The shapes of the laterally varying sediment density polygons derived from well and seismic data were modified to include structural warping contained in the seismic data.

Modification of the base salt geometry. Little or no modification was implemented where the base of salt was easily interpreted from the seismic data. Large modifications were made to the base of salt where the seismic data provided no clear indication as to the location of the salt base.

Incorporation of a low velocity, low density, "gumbo" zone below the tabular salt. The geometry of the "gumbo" zone was derived from subtle amplitude indications in the seismic image and correlations between the calculated and observed gravity fields.

The final depth model (Fig. 12 and Fig. 13) displays a high degree of fit between the observed and calculated gravity field derived from the integrated model. The nearest to surfacesteeply dipping top of salt has been modified slightly from that of the original seismic-derived interpretation in order to match the high frequency component of the observed. This was deemed to be acceptable by the explorationist as the seismic method may indicate inaccurate dips for near vertical structures.

The original base of salt, as interpreted from the seismic data alone and displayed in Fig. 12, differs from the base of salt derived from the integrated approach over portions of the body. The discrepancy between the original and final models is as much 3,500 feet. Also notable is the large portion of the model over which the integrated model confirms the initial seismic derived base salt interpretation.

Although high-resolution magnetic data was not used in this case study its application would have provided an additional inexpensive data set for the imaging of the complex salt feature.

Case Study 3:
North Sea Magnetics - Facies Location Mapping
As demonstrated in a seismic time slice from the Southern Gas Basin of the North Sea (Fig. 14), mapping of an important chalk subcrop boundary is difficult due to a significant velocity boundary between the chalk and the surrounding section. Identification of this boundary is essential in determining proper seismic velocities to most accurately image the underlying gas bearing sands. Using the high-pass filtered and enhanced magnetic data, the chalk boundary is clearly indicated in map view (Fig. 15). In addition, a likely fault-induced lateral offset in the boundary is clearly indicated. When merged with the results of the gravity and seismic interpretation (Fig. 16) and wells, which indicate multiple faults across the area, many new insights can be gained about the integrity and continuity of the reservoir.

By incorporating high resolution gravity into 3D seismic surveys and interpretations, there is a positive impact on the final interpreted results. The cost effective high resolution data can be used to constrain and verify seismic velocities, particularly in problem areas near complex structures and seismic shadow zones.

The mechanics of integration have been simplified by the development of real-time, integrated workstation applications.

We thank Jack Weyand of Sidney Schafer & Associates for his work on the density models; Bill Gray and Corine Prieto of IGC for their seismic velocity grids and density conversion; TGS and Geco-Prakla for their top/base salt interpretation and VDIP velocity grid, GDC for well data and compilation, David Harrison and Mobil North Sea Limited for the data and interpretation, and Elizabeth Johnson and Mark Odegard of Unocal Corporation for their Gulf of Mexico data and interpretation.

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LaFehr, T.R., Valliant, H.D., and MacQueen, J.D. "High-Resolution Marine Gravity by Digital Control", paper presented at the SEG Annual Conference and Exhibition, New Orleans, 1992.

Valliant, H.D., Halpenny, J. and Cooper, R.V. "A microprocessor-based controller and data acquisition system for LaCoste & Romberg air-sea gravity meters" Geophysics, 50, 840-845, 1985.

Saad, A.H. "Interactive Integrated Interpretation of Gravity, Magnetic, and Seismic Data: Tools and Examples." Offshore Technology Conference Proceedings, Paper 7079, 35-44, 1993.

Pawlowski, R. "Emerging Workstation-based Potential Field Methodologies." The Leading Edge (SEG), June, 1994, 687-689, 1994.

Bain, J.E., Weyand, J., Horscroft, T.R., Saad, A.H., and Bulling, D.N. "Complex Salt Features Resolved by Integrating Seismic, Gravity, and Magnetics." EAEG/EAPG 1993 Annual Conference and Exhibition, expanded abstracts.

A New Dimension in 3D Brian S. Anderson and Mark E. Weber, Fugro-LCT, Inc.

A New Dimension in 3D

Brian S. Anderson and Mark E. Weber, Fugro-LCT, Inc.